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Title: Bifurcation physics of magnetic islands and stochasticity explored by heat pulse propagation studies in toroidal plasmas

Abstract

Bifurcation physics of the magnetic island was investigated using the heat pulse propagation technique produced by the modulation of electron cyclotron heating. There are two types of bifurcation phenomena observed in LHD and DIII-D. One is a bifurcation of the magnetic topology between nested and stochastic fields. The nested state is characterized by the bi-directional (inward and outward) propagation of the heat pulse with slow propagation speed. The stochastic state is characterized by the fast propagation of the heat pulse with electron temperature flattening. The other bifurcation is between magnetic island with larger thermal diffusivity and that with smaller thermal diffusivity. The damping of toroidal flow is observed at the O-point of the magnetic island both in helical plasmas and in tokamak plasmas during a mode locking phase with strong flow shears at the boundary of the magnetic island. Associated with the stochastization of the magnetic field, the abrupt damping of toroidal flow is observed in LHD. The toroidal flow shear shows a linear decay, while the ion temperature gradient shows an exponential decay. Lastly, this observation suggests that this flow damping is due to the change in the non-diffusive term of momentum transport.

Authors:
 [1];  [2];  [2];  [1];  [1];  [3];  [1];  [2];  [4];  [2]
  1. National Inst. for Fusion Science, Toki (Japan); Graduate Univ. for Advanced Studies, Toki (Japan)
  2. National Inst. for Fusion Science, Toki (Japan)
  3. General Atomics, San Diego, CA (United States)
  4. Kyushu Univ., Kasuga (Japan). Research Inst. for Applied Mechanics
Publication Date:
Research Org.:
General Atomics, San Diego, CA (United States)
Sponsoring Org.:
USDOE; National Institute for Fusion Science (NIFS-NINS)
OSTI Identifier:
1375943
Grant/Contract Number:
FC02-04ER54698; FG03-97ER54415; AC05-00OR22725
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Nuclear Fusion
Additional Journal Information:
Journal Volume: 56; Journal Issue: 9; Journal ID: ISSN 0029-5515
Publisher:
IOP Science
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; stochasticity; magnetic island; bifurcation; heat pulse propagation

Citation Formats

Ida, K., Kobayashi, T., Yoshinuma, M., Suzuki, Y., Narushima, Y., Evans, T. E., Ohdachi, S., Tsuchiya, H., Inagaki, S., and Itoh, K. Bifurcation physics of magnetic islands and stochasticity explored by heat pulse propagation studies in toroidal plasmas. United States: N. p., 2016. Web. doi:10.1088/0029-5515/56/9/092001.
Ida, K., Kobayashi, T., Yoshinuma, M., Suzuki, Y., Narushima, Y., Evans, T. E., Ohdachi, S., Tsuchiya, H., Inagaki, S., & Itoh, K. Bifurcation physics of magnetic islands and stochasticity explored by heat pulse propagation studies in toroidal plasmas. United States. doi:10.1088/0029-5515/56/9/092001.
Ida, K., Kobayashi, T., Yoshinuma, M., Suzuki, Y., Narushima, Y., Evans, T. E., Ohdachi, S., Tsuchiya, H., Inagaki, S., and Itoh, K. 2016. "Bifurcation physics of magnetic islands and stochasticity explored by heat pulse propagation studies in toroidal plasmas". United States. doi:10.1088/0029-5515/56/9/092001. https://www.osti.gov/servlets/purl/1375943.
@article{osti_1375943,
title = {Bifurcation physics of magnetic islands and stochasticity explored by heat pulse propagation studies in toroidal plasmas},
author = {Ida, K. and Kobayashi, T. and Yoshinuma, M. and Suzuki, Y. and Narushima, Y. and Evans, T. E. and Ohdachi, S. and Tsuchiya, H. and Inagaki, S. and Itoh, K.},
abstractNote = {Bifurcation physics of the magnetic island was investigated using the heat pulse propagation technique produced by the modulation of electron cyclotron heating. There are two types of bifurcation phenomena observed in LHD and DIII-D. One is a bifurcation of the magnetic topology between nested and stochastic fields. The nested state is characterized by the bi-directional (inward and outward) propagation of the heat pulse with slow propagation speed. The stochastic state is characterized by the fast propagation of the heat pulse with electron temperature flattening. The other bifurcation is between magnetic island with larger thermal diffusivity and that with smaller thermal diffusivity. The damping of toroidal flow is observed at the O-point of the magnetic island both in helical plasmas and in tokamak plasmas during a mode locking phase with strong flow shears at the boundary of the magnetic island. Associated with the stochastization of the magnetic field, the abrupt damping of toroidal flow is observed in LHD. The toroidal flow shear shows a linear decay, while the ion temperature gradient shows an exponential decay. Lastly, this observation suggests that this flow damping is due to the change in the non-diffusive term of momentum transport.},
doi = {10.1088/0029-5515/56/9/092001},
journal = {Nuclear Fusion},
number = 9,
volume = 56,
place = {United States},
year = 2016,
month = 7
}

Journal Article:
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  • The impact of sheared toroidal rotation on the evolution of pressure driven magnetic islands in tokamak plasmas is investigated using a resistive magnetohydrodynamics model augmented by a neoclassical Ohm's law. Particular attention is paid to the asymptotic matching data as the Mercier indices are altered in the presence of sheared flow. Analysis of the nonlinear island Grad-Shafranov equation shows that sheared flows tend to amplify the stabilizing pressure/curvature contribution to pressure driven islands in toroidal tokamaks relative to the island bootstrap current contribution. As such, sheared toroidal rotation tends to reduce saturated magnetic island widths.
  • This is a study of the effect of an energetic ion population injected from a neutral beam source on the nonlinear stability of magnetic islands. A kinetic theory is used to obtain the island current, which includes previously known modifications resulting from bootstrap currents and resistive interchanges. The island current thus obtained is used in an asymptotic treatment of the perturbed Ampere's law, which yields an evolution equation for the island width. It is shown that the island width saturates at a very small amplitude in tokamaks (with {Delta}{prime}{gt}0) when the energetic ion density profile peaks just outside the rationalmore » surface. Energetic ions can also be used to modify the island width in three-dimensional equilibria. It is found that in stellarator equilibria with large vacuum islands, energetic ions can be injected to reduce the widths of equilibrium islands.« less
  • The coupling between the transport and magnetic topology is an important issue because the structure of magnetic islands, embedded in a toroidal equilibrium field, depends on the nature of the transport at the edge of the islands. Measurements of modulated heat pulse propagation in the DIII-D tokamak have revealed the existence of self-regulated oscillations in the radial energy transport into magnetic islands that are indicative of bifurcations in the island structure and transport near the q = 2 surface. Large amplitude heat pulses are seen in one state followed by small amplitude pulses later in the discharge resulting in amore » repeating cycle of island states. These two states are interpreted as a bifurcation of magnetic island with high and low heat pulse accessibility. In conclusion, this report describes the discovery of a bifurcation in the coupled dynamics between the transport and topology of magnetic islands in tokamak plasmas.« less
  • The coupling between the transport and magnetic topology is an important issue because the structure of magnetic islands, embedded in a toroidal equilibrium field, depends on the nature of the transport at the edge of the islands. Measurements of modulated heat pulse propagation in the DIII-D tokamak have revealed the existence of self-regulated oscillations in the radial energy transport into magnetic islands that are indicative of bifurcations in the island structure and transport near the q = 2 surface. Large amplitude heat pulses are seen in one state followed by small amplitude pulses later in the discharge resulting in amore » repeating cycle of island states. These two states are interpreted as a bifurcation of magnetic island with high and low heat pulse accessibility. In conclusion, this report describes the discovery of a bifurcation in the coupled dynamics between the transport and topology of magnetic islands in tokamak plasmas.« less